The photovoltaic (PV) industry is growing rapidly and is responsible for an increasing amount of silicon being consumed beyond the more traditional uses as integrated circuit (IC) applications. Today, the silicon needs of the solar cell industry are partially competing with the silicon needs of the IC industry. With present manufacturing technologies, both IC and PV industries require a refined, purified, silicon feedstock as a raw silicon starting material.
Materials alternatives for the bulk of current solar cells range from mono-crystalline silicon wafers, for example based on very clean raw silicon such as electronic-grade (EG) silicon feedstock needed for the IC industry, to multi-crystalline (mc) silicon wafers based on not as clean raw silicon such as the so-called solar-grade (SOG) silicon feedstock or an even lower-quality material called upgraded metallurgical-grade (UMG) silicon feedstock.
Low-grade feedstock materials for the PV industry, such as UMG silicon, are typically processed into ingots and wafers of mc-silicon where the ultimate, solar cell relevant quality is typically controlled by grain boundaries, other structural defects and a relatively high concentration of impurities such as transition metals. Also carbon related and oxygen related defects in the wafer bulk can degrade cell properties, in particular when associated with metals. Some species of the broad defect spectrum may be passivated with hydrogen to reduce their electrical degradation potential.
Higher-grade feedstock materials for the solar cell industry, such as EG silicon, are typically processed into mono-crystals and, subsequently, wafers with mono-crystalline structure where the ultimate, solar cell relevant quality is controlled by impurities similar to the case of mc-silicon described above. There are two well-established growing techniques for mono-crystals of silicon (in the following called crystals). By far dominant is the Czochralski (CZ) technique where a CZ crystal is pulled out of a silicon melt residing in a quartz crucible. Medium to high-grade feedstock silicon is employed for generating the CZ silicon melt. A more sophisticated alternative is the Floating Zone (FZ) technique where a FZ crystal is grown by “floating” a small melt zone through a so-called supply rod of high-grade feedstock silicon. One way of getting predetermined amounts of elements into FZ crystals is so-called “pill doping” into supply rods before generating the melt zone. Typically, FZ silicon crystals contain less impurities than CZ crystals, mainly because no crucible is required.
In any case, since silicon is brittle at room temperature, there is the general problem of wafer breakage at wafer and solar cell processing and handling, including the manufacturing of modules out of solar cells. Consequently, the mechanical strength of silicon wafers and related solar cells is also an important quality factor in the PV industry, besides electrical properties. This holds for mono-crystalline material and equally for multi-crystalline ingot material.
Wafer breakage is initiated by crack formation and subsequent propagation. Cracks may originate from, for example, handling-induced local damage on surfaces, in particular at edges and corners. State-of-the-art solar cell manufacturing technology uses careful handling and processing of wafers and solar cells to avoid such situations. Intrinsic material strength of bulk silicon is also a function of bulk lattice defects. Of particular concern are defects that generate local tensile lattice strain, enabling internal crack formation/propagation at reduced external force (relative to an ideal lattice structure).
A need exists for a simple process that delivers UMG-based multi-crystalline silicon material with good ingot yield and improved mechanical and electrical properties, the latter in regard to solar cell quality. Such a process should be easily transferable to higher-grade, non-UMG feedstock silicon which is used partially or exclusively for producing mono-crystalline silicon materials, for example by applying the CZ technique or the FZ technique.